This invention relates to time division multiple access (TDMA) communications systems, and in particular, to a method and architecture for a TDMA receiver incorporating a unique word correlation control loop.
In a TDMA communications system, a number of transmitters can transmit on the same frequency channel, but at different times. A remote receiver for receiving a particular transmitted signal knows before hand at what approximate time the transmitted signal will occur and is enabled only during that time. Details for such a TDMA communications system are provided in U.S. Pat. No. 5,598,419, which is incorporated by reference herein.
The TDMA technique makes very efficient use of the frequency spectrum since multiple users may use the same radio-frequency (RF) channel at the same time without interfering with one another. FIG. 1 illustrates one type of TDMA system where a plurality of remote portable stations 10, 11, 12, and 13 share a same frequency channel while transmitting to and receiving from a base station 14. The portable stations 10-13 can be, for example, wireless telephones, and the base station 14 can be a high-power transponder base cell. All communications between portable stations 10-13 are routed through base station 14. For communication, each of portable stations 10-13 and base station 14 contains a transceiver which operates in various modes, such as set-up/control mode, transmit/receive mode, data mode, etc.
One embodiment for such a TDMA system is the Personal Handy Phone System (PHS), the requirements for which are described in RCR Standard-28, incorporated herein by reference. As an alternate embodiment, such a TDMA system may be the ETSI DECT standard, also incorporated herein by reference. Further, slow frequency hopping systems, compliant with C.F.R. Title 47, part 15, and intended for the U.S. ISM-bands, may be derived from the aforementioned formal standards.
In a TDMA system, each remote transceiver (e.g., portable stations 10-13), when active, is allocated certain times slots within which it may transmit a bursted signal or receive a bursted signal. FIG. 2 illustrates a frame 16 containing slots 0 through 7, where frame 16 is repeated on a signal frequency channel. The period of frame 16 may be, for example, 5 msecs. Assuming all four portable stations 10-13 in FIG. 1 are being actively used at the same time, portable stations 10, 11, 12, and 13 may be allocated slots 0, 1, 2, and 3, respectively, for transmitting bursted signals to base station 14, while portable stations 10, 11, 12, and 13 may be allocated slots 4, 5, 6, and 7, respectively, for receiving bursted signals from base station 14. The amount of information stored in each of portable stations 10-13 during a frame period is transmitted in a burst within a single slot. In one embodiment, the bit rate of the transmitted bits in a slot is approximately 384K bits per second, and the corresponding symbol rate is, therefore, 192K symbols per second.
A sample protocol 18 which dictates the information required to be transmitted during a single slot is also shown in FIG. 2. Protocol 18 may consist of a ramp-up (R) field 20, a start symbol (SS) field 21, a preamble and/or clock recovery field 22, a unique word or slot synchronization field 23, a data field 24 (typically used as a traffic channel or TCH), a CRC (for error correction and verification) field 25, and a guard bits field 26. The lengths and types of fields in a protocol vary depending on the mode of the transceiver (e.g., set-up/control mode, transmit/receive mode, etc.). While in the traffic mode, where voice is to be transmitted, data field 24 contains audio or voice data.
Modern, digital TDMA communication systems require very accurate synchronization in the time domain. To achieve this, such systems commonly employ a known marker sequence within the TDMA burst architecture. In the case of the conventional TDMA system (such as the PHS) described above, the xe2x80x9cunique wordxe2x80x9d of protocol 18 is used as the marker sequence for each burst. The unique word comprises a predefined sequence of bits and is chosen to have special orthogonal properties which yield a sharp peak during an auto-correlation process performed at a transceiver. When the marker sequence is detected by the transceiver, the transceiver""s time-base is re-aligned to the incoming signal. Thus, the transceiver completely re-synchronizes on each frame. Afterwards, the payload data in the burst can be properly recovered.
Generally, a marker sequence is xe2x80x9cdetectedxe2x80x9d by comparing and correlating certain bits in a slot or incoming burst of data against the predefined sequence for the unique word. If the bits in the incoming burst exactly match respective bits in the predefined sequence, the unique word is considered to be xe2x80x9cdetectedxe2x80x9d for the slot.
However, if environmental conditions (e.g., noise or fading) exist which adversely affect the RF channel, some of the bits within a transmitted signal may be corrupted, thus preventing exact matching of the appropriate bits in a received burst with respective bits in the predefined sequence. To account for this, previously developed TDMA systems allow some error during correlation. In particular, with such systems, a unique word is still considered to be xe2x80x9cdetectedxe2x80x9d even though one or more bits in a received signal do not exactly match the respective bits in the predefined sequence. Each un-matched bit constitutes an xe2x80x9cerror.xe2x80x9d
With any given previously developed TDMA system, the number of errors allowed during correlation is a pre-set and unchanging value. This inflexibility prevents the system from adjusting with changes in environmental conditions and other factors. That is, the number of allowable errors for near ideal conditions is the same as the number of errors allowed for conditions in which noise or fading adversely affect the RF channel. This is problematic in that a smaller margin of error should be afforded when conditions are ideal, and a greater margin of error should be afforded when conditions are other than ideal.
What is needed is a more flexible technique wherein the number of errors allowed during unique word correlation may be dynamically changed, for example, to account for changes in environmental conditions.
A method and architecture for a TDMA receiver incorporating a control loop for the correlation of a unique word is provided which substantially improves the detection of unique words and reduces the number of false detections, thereby improving the quality of voice in a TDMA system.
In accordance with an embodiment of the present invention, a TDMA receiver architecture includes a unique word correlators control loop. The unique word correlators control loop is programmable and controls or adjusts of the number of errors allowed during correlation of the unique word. For a given frame, the number of allowable correlation errors can be set based upon various parameters, including, for example, received signal strength indicator (RSSI) level, preamble detection, CRC detection, timebase status, channel type, communication burst type, and unique word correlation result for the current frame or a previous frame.
PHS utilizes four different unique words to achieve time synchronization in the TDMA system. Specifically, two 32-bit unique words may be provided for control channel synchronization, and two 16-bit unique words may be provided for communication channel synchronization. For each channel type (control or communication), one of the respective unique words is used for uplink (i.e., transmission from portable station to base station), and the other unique word is used for downlink (i.e., transmission from base station to portable station). The automatic unique word detection mechanism described herein works on all four unique words.
According to one embodiment of the present invention, a system includes a unique word correlator module which correlates a unique word field in a burst of a time division multiple access (TDMA) signal against a predefined sequence. Automatic timing control circuitry is coupled to the unique word correlator module. The automatic timing control circuitry derives a number of errors that are allowable during correlation of the unique word field.
According to another embodiment of the present invention, a method includes the following steps: receiving a burst of a time division multiple access (TDMA) signal, the burst having a unique word field; correlating the unique word field against a predefined sequence; and deriving a number of errors that are allowable during correlation of the unique word field.
An important technical advantage of the present invention includes dynamically changing the number of allowable errors for correlation of a unique word in order to adjust for changing conditions. Accordingly, the present invention improves the quality of voice communication in a TDMA system, thereby promoting customer satisfaction and loyalty for a provider. Other important technical advantages are readily apparent to one skilled in the art from the following figures, descriptions, and claims.